In typical U.S. digital cellular telephone systems, analog voice signals are sampled and converted to a digital bit stream. In order to save bandwidth and therefore provide economic advantage, the digital bit stream is compressed by a source encoder before transmission across a radio channel. Generally, cellular systems provide for either full-rate or half-rate encoding of voice signals.
Full-rate source encoding in a typical GSM (Global System for Mobile Communications) system utilizes an LPC (Linear Prediction Coding) encoder with long-term prediction and regular pulse excitation. The output of the LPC encoder is generally 260 bits every 20 milliseconds, to give an encoding rate of 13 kbits/s. The 13 kbits/s signal output by the LPC encoder is input to a signal expander, for example, a channel encoder, which adds redundancy to the bit stream and thereby increases its rate to 22.8 kbits/s. The purpose of the redundancy is to minimize the consequence of bit errors that are often induced by a noisy radio channel.
Half-rate GSM source encoding utilizes a VSELP (Vector Sum Excited Linear Prediction) encoder whose output is generally 112 bits every 20 milliseconds, to give an encoding rate of 5.6 kbits/s. The 5.6 kbits/s signal output by the VSELP encoder is expanded by a channel encoder, which adds redundancy to the bit stream and outputs 228 bits every 20 milliseconds, thus increasing the bit rate from 5.6 kbits/s to 11.4 kbits/s. Again, the purpose of the added redundancy is to minimize transmission errors.
Half-rate encoding effectively doubles the capacity of a cellular system. Accordingly, established cellular standards provide half-rate capability as an economic benefit (twice as many revenue generating users). However, there is some penalty to be paid in return, namely, lower bit rate source encoding, e.g., half-rate encoding, goes hand-in-hand with reduced audio fidelity.
The above comparison between full-rate source encoding and half-rate source encoding assumes that the RF (Radio Frequency) channel carrying the encoded signals does not introduce transmission errors beyond the correction capability of the receiver receiving such signals. In general, this is a good assumption because cellular systems are typically designed to provide a relatively high SNR (Signal-to-Noise Ratio), and therefore a relatively low BER (Bit-Error-Rate). However, under conditions of low SNR, a low-bit-rate encoded signal, whether half-rate or full-rate, can suffer severely degraded audio quality.
This breakdown in performance is important in practice, as commercial applications are being developed where a radio system, conforming to an established cellular-radio air-interface standard, fails to provide the relatively high SNR on which the half-rate/full-rate trade is premised, or which fails to provide adequate SNR for the effective operation of either full or half-rate encoding. Such commercial applications include, but are not limited to, satellite communication systems with a cellular-standard air-interface which inherently has low link margins, as well as extended-range cellular systems that provide marine telephone service or telephone service capable of penetrating an office building, or the like. Moreover, cellular systems often experience episodes of significantly reduced SNR caused by channel fading and shadowing, which may result in degraded audio quality.
The present invention is directed toward overcoming one or more of the above-mentioned problems.